The present invention relates to a rotary yarn twisting disc of the type adapted for imparting false twist to an advancing yarn. Twisting discs of the described type are known, and are described for example in U.S. Pat. Nos. 4,115,987 and 3,901,011. In the disclosures of these patents, the discs are used in friction false twist units having three shafts, which are arranged at the corners of an equilateral triangle, and which rotate in the same direction. The discs are mounted to the shafts in such a way that they overlap above the center of the triangle, and the discs form a zigzag operative yarn path of travel extending axially therebetween.
One advantage of the described yarn twisting discs is the fact that they twist and concurrently advance the yarn. As a result, the twisting discs, and the false twisting units in which they are used, may be used for high yarn speeds with a high degree of twist insertion.
The degree of twist and the effect of advancing are theoretically correlated to each other by the angle between the yarn and the tangent to the disc. In static state this angle is the same from tile entrance of the yarn to the exit of the disc. The static angle may be predetermined by the separation of the shafts, the diameter of the discs, and the axial separation of the discs. The tangent of this angle defines the relationship between advance and twist. For setting the apparatus into operation, this theoretical relationship first has to be defined, which is referred to herein as the "predetermined dependence". It has been observed that in addition to the mechanically predetermined dependence, a non-determinable dependence exists between the twist effect and conveying effect, thereby providing a limitation for the yarn speed at a given twist level. It is an object of the present invention to avoid this limitation.